As electronic technology advances, high efficiency and stable operation have become important design considerations. Isolation transformers can be used for adapters, chargers, and/or other systems that require a high level of safety. Such a system can include a primary-side circuit and a secondary-side circuit separated from each other by an isolation transformer.
The operation of the system includes a constant current charging control process and a constant voltage control process. The relationship of the output voltage and output current from the system is illustrated in FIG. 1. During interval B, the load is charged. When the output voltage Vo is less than an output voltage threshold Vth, the transformer maintains output current Io at the vicinity of the upper threshold value Ith while varying the output voltage Vo to increase the output voltage Vo to the output voltage threshold Vth. As a result, the control is based on constant current during interval B. During interval A, when the output voltage reaches the output voltage threshold Vth, the transformer enters a normal operation mode in which the output voltage Vo is maintained while the output current Io is varied. To achieve constant current control, the current at the output terminals are monitored and the switch on the primary-side is controlled based on the monitored output current in a feedback fashion.
Typically, a current monitoring circuit on the secondary-side is used for current control by obtaining a signal with an optical coupler. However, such an implantation is structurally complex, energy consuming, and inefficient. Other current monitoring circuits that do not utilize the secondary-side monitoring typically utilize complex models based on monitoring input voltage, output diode on duration, and the peak current limit on the primary-side circuit. Accordingly, there is a need for improved control circuits with high efficiency and low structural complexity.